1. Field of the Invention
The present invention relates to the de-NOx purification process where NOx in the exhaust gas is reduced in the presence of stoichiometrically excess amount of oxygen, the exhaust gas purification process where soluble organic substance in the exhaust gas from a diesel engine is purified, and catalytic converters used in the above processes.
2. Description of Related Arts
Catalytic converters have been typical measures for purifying the exhaust gas emitted from a vehicle engine. As FIG. 17 shows, the conventional catalytic converter comprises a housing 95 and a catalytic layer 91 disposed in the housing 95. The catalytic layer 91 is a ceramics or metal carrier (monolith) of honeycomb type loading catalytic component such as platinum, palladium, and the like. The length of the carrier available for the vehicle is typically in the range from several 10s to 300 mm.
In the case of "oxidation catalyst" for purifying HC, CO and the like in the exhaust gas, and "three way catalyst" for oxidizing HC and CO concurrently with reducing NOx in the exhaust gas in theoretical mixture combustion, the higher initial temperature of the gas admitted into the catalytic layer will accelerate the reaction and improve the conversion efficiency.
In the above both cases, the upper limit of the initial gas temperature is determined based on the heat-resistance property of materials used as the carrier and catalyst constituting the catalytic layer, rather than reactions.
In the case of "de-NOx catalyst" for reducing NOx in the exhaust gas containing stoichiometrically excess amount of oxygen, the initial gas temperature is so kept to bring the conversion efficiency to a peak value. If the temperature is further increased, the conversion efficiency will decline because the NOx purification ability reduces. The gas temperature for the peak conversion efficiency varies depending upon what type of reductants and catalyst are used. For example, when using H.sub.2 as the reductant, the temperature will be in the range from 90.degree. to 130.degree. C . When using the material selected from HC group such as propylene as the reduction agent, it will be in the range from 250.degree. to 350.degree. C. Further increase in the temperature will deteriorate the purification ability.
However, unless the high temperature destroys the carrier-forming material, the purification ability can be restored when the initial gas temperature lowers again. With the three way catalyst, the NOx conversion efficiency reaches approximately 80% or more. In contrast, with the de-NOx purification in the presence of stoichiometrically excess amount of O.sub.2, the ratio is as low as 30 to 50% or less.
Followings are considered to be relevant factors to achieve the peak conversion efficiency at a specific temperature in the de-NOx purification process.
(a) Catalytic component such as platinum is effective for both reactions of reduction (between reductant and NOx) and oxidation (between reductant and O.sub.2). PA1 (b) Both reactions occur almost simultaneously. The temperature will affect as to which reaction is predominant. PA1 (c) The exhaust gas in the presence of stoichiometrically excess amount of O.sub.2 tends to have substantially higher O.sub.2 concentration (2-3% or more) than NOx concentration (2,000-3,000 PPM or less). Increasing the amount of the reductant for higher conversion efficiency will facilitate the oxidation between O.sub.2 and reductant. Then exothermic heat of the oxidation will lead to increasing the catalytic temperature, resulting in undesirable cycle for further facilitating oxidation. PA1 (d) Removing O.sub.2 from the exhaust gas (keeping the concentration of other components such as NOx intact) increased NOx conversion efficiency to a considerably high value, which is equivalent to or more than that of the three way catalyst. PA1 (e) Increasing the catalytic volume by lengthening the catalytic layer (carrier) along the exhaust gas flow does not increase the conversion efficiency, unlike the three way catalyst. PA1 (f) In the case of the exhaust gas containing NOx and O.sub.2, the catalytic temperature increased sharply at the point going down by several millimeters from the inlet to the catalytic layer. PA1 (g) On the contrary to (d), removing NOx exhibited the same tendency as in (f), sharp increase in the temperature at the same point. PA1 (h) The reaction between NOx and H.sub.2 occurred very quickly at the point going down by several millimeters from the inlet to the catalytic layer. After passing such point, the reaction between O.sub.2 and H.sub.2 occurred predominantly to inhibit reduction between NOx and H.sub.2.
After conducting various examinations with respect to the peak conversion efficiency, the following findings were obtained through the de-NOx purification process using H.sub.2 as the reductant.
The structure of catalytic converter used for the de-NOx catalyst is basically the same as those used for "catalytic oxidation" and "three way catalyst" except that the noble metal to be carried is variable depending on the usage. However, the respective reactions at the catalyst are greatly different from each other. Especially the NOx catalyst fails to attain sufficient conversion efficiency, without effective process for the de-NOx purification.
The exhaust gas emitted from the diesel engine contains sulfur compound such as SO.sub.2 as well as soluble organic substance such as hydrocarbons. When removing the soluble organic substance through oxidation, the exothermic heat of the oxidation will increase the exhaust gas temperature. Excessive increase in the temperature may oxidize the sulfur compound to form sulfate such as SO.sub.3, H.sub.2 SO.sub.4 and the like, which are air pollutants. (see Embodiment 7)
It is necessary to provide-the effective process for purifying the soluble organic substance without forming the sulfate. The soluble organic substance is likely to be produced especially when operating the diesel engine at low speeds and low loads.